Which one is beter and why? Kira looks a bit crazy and hectic to play someone like sparks with the low hp and movement speed, as where Arty looks a bit more solid with higher hp and lower speed.
Could Arty be played in replacement of a Skyhammer/Fragger? A slower walk in kill them all type of Merc with a healer behind you?
Kira looks a bit too much effort with Dreiss but i could be wrong?
In my experience Arty requires a very different playstyle than Skyhammer even though they have the same HP and speed mainly because of the primary weapons. Arty has only burst rifles (br-16 and stark) and the dreiss, all guns that are very difficult to use in close quarters while Skyhammer has two full auto assault rifles (m4 and timik) which are way more forgiving. Also, Arty’s ability (very underrated IMO) is very useful when far from the frontlines, much more unpredictable than Skyhammer’s airstrike which is only deployable in a limited range of a nade throw.
As for using him as replacement of Fragger, don’t even think about it.
Finally, regarding Kira, I like her a lot. She has more or less the same playstyle of Arty, but:
Personally, if you are unsure about which one to buy first, I would advice Kira because I found her more easy to use when I started out playing. I hope my post has been helpful. 
I agree with sharpSubmarine on the second point. Kira’s speed is a huge advantage. Maybe it’s the way I play, but I find HP doesn’t help me too much. Speed definitely does. She also has a sweet loadout with the br, stilleto, and chopper and larger AOE augments. AOE makes the laser a bit better, and chopper makes the stilleto heavy attack damage a bit ridiculous.
Anyway, the speed lets you dodge and strafe better, cover ground to get to objectives, and just generally makes you harder to hit.
Don’t bother with Arty, as he is considerably harder to play then some other mercs, and will acquire some readjusting. I can only recommend Arty if:
You are amazing at headshots. I mean REALLY good.
You have a good knowledge of the maps.
You have other mercs you can fall back on.
I prefer Arty, more HP make him better duelist, the speed doesn’t help to aim with kira
I am usually not very good with Dreiss and tend to miss more headshots than I hit. For this reason I like Kira because I’d rather be fast, have a more useful ability, and not get shot back when I fail to kill someone with my primary.
Well, you can either use a laser beam that can easily kill entire teams with a single use or you can use an artillery strike that almost never 1 hits anyone. Sooo yeeeaaah…
This is a tough one, simply because you’re trying to compare an apple with a driess to an orange with a driess…think I messed that up.
Kira - mover and shaker, her highly visible laser is designed to scare and confuse, in the hands of an experienced/lucky player she can wipe out an entire team in one go, if not, she gets to watch those roaches scatter. High speed, low hp, she sticks with the team where her ammo station does the most good.
This speed means that the driess is that much harder to aim and a burst rifle is much more forgiving for her, trading high dmg per shot for more shots on target.
Arty- Suppression specialist, his job is sneaky. His Artillery is hard to spot and unfortunately, hard to aim due to bugs, However if you need to prevent c4 from being planted/defused, he can drop artillery right on that spot from the other side of the map.
Low speed and higher hp, hes a meduim range duelist as well with the driess. Snipers cant 1v1 him, and cant hide behind cover without artillery spattering them all over the place. Experienced players will stay out of the limelight, sneaking around the edges and stopping any rushes dead in their tracks.
With a good team covering the flanks, Arty is a force to be reckoned with. You cant group behind cover, and staying in the open is dangerous . You go to plant plant/defuse,either you get head shot from a distance or artillery turns you into a red stain.